bridge: prevent flooding IPv6 packets that do not have a listener

Currently if there is no listener for a certain group then IPv6 packets
for that group are flooded on all ports, even though there might be no
host and router interested in it on a port.

With this commit they are only forwarded to ports with a multicast
router.

Just like commit bd4265fe ("bridge: Only flood unregistered groups
to routers") did for IPv4, let's do the same for IPv6 with the same
reasoning.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>